The commonly held view is that the process used by Robert Adamson for his calotype work with David Octavius Hill is unknown because Adamson wrote nothing down and died very young, at the age of 26, in 1848.
However, a paper published anonymously in Chamber’s Information for the People, Vol II, 1849, describes an improved method of the Talbotype using William Furlong’s single-wash iodising technique which it gives as dating from 1844. The author of this paper has been identified as Dr. John Adamson by Dr Sara Stevenson and I found this out via private communication between Dr Stevenson, Professor Roger Taylor, Dr. Mike Ware and picked up by Rob Douglas in the Calotype Society. Sara Stevenson points out that the attribution is made in Dr. John Adamson’s obituary. As Adamson was writing in 1849, just a year after his brother died, it is fair to assume that he is describing the process his brother used together with D.O. Hill.
The process itself was discovered by William Holland Furlong in 1843. In their paper “Pilgrims of the Sun” [History of Photography, Volume 27, Winter 2003], Taylor & Ware tell us that in 1841 Furlong was assistant to the Professor of Chemistry, Arthur Connell, at the University of St Andrews. Furlong met Sir David Brewster who was a friend of Talbot. In 1841 Furlong was in touch with Talbot directly asking for instructions himself. On April 3rd 1843, John Adamson read a paper before the Literary & Philosophical Society of St Andrews in which he outlined Furlong’s proposed improvement on Talbot’s method by using the single-wash method.
In Talbot’s original process the paper is first washed over with silver nitrate solution and dried. Then it is washed over with potassium iodide solution producing silver-iodide in the paper. There follows a washing step that is time-critical and the process is difficult to control. Furlong’s method avoids the problems associated with Talbot’s manipulation.
Thomas Sutton wrote a pamphlet “The Calotype Process; a Handbook to Photography on Paper” published by Joseph Cundall in 1855. A letter by Furlong was addressed to Sutton in response to the latter’s request that Furlong elaborate on his experiences (Photographic Notes, vol I, 1856, p5). In the letter, Furlong describes how he stumbled on the idea of the single-wash whilst struggling with Talbot’s double-wash and he says he published it in 1841 although he probably got the date wrong there as it was more likely 1842-3.
In Furlong’s technique, silver nitrate is first dissolved in distilled water and then potassium iodide is added in an increasing amount. At first a yellow precipitate of silver iodide is thrown down and as you add more and more potassium iodide the solution turns milky-white before clearing when a critical balance is achieved which redissolves the silver iodide. This clear solution is then used to wash the paper which is then dried. Further washing of the dried paper causes the silver iodide to re-precipitate into the fibres of the paper.
Sutton advocates washing the precipitate before continuing to add a final amount of potassium iodide and fresh distilled water to complete the re-dissolution. But Furlong says “I have never found the slightest advantage in first precipitating the iodide of silver and then washing it”.
Furlong (Adamson) then excites the paper using only aceto-nitrate (no gallic acid) and then develops the image after exposure using equal parts of aceto-nitrate and saturated gallic acid solution.
Adamson’s recipe for the “double-iodide” solution (expressed in modern measurements and scaled for 300ml volume) is:
17.1g silver nitrate 143.7g potassium iodide 300ml distilled water
So we have an account by Dr John Adamson of the process that the early Scottish calotype circle were using. Since Robert Adamson learnt photography from his brother then it is highly likely this is the method that Robert Adamson also used.